1. Field of the Invention
The present invention relates to solid oxide fuel cells.
2. Related Art Statement
Recently, fuel cells have been recognized as useful for power generating equipment. The fuel cell is an a device capable of directly converting chemical energy possessed by fuel to electric energy. Since the fuel cell is free from limitation of Carnot's cycle, the cell is an extremely promising technique in that the fuel cell essentially has a high energy conversion efficiency, a variety of fuels (naphtha, natural gas, methanol, coal reformed gas, heavy oil, etc.) may be used, the cell provokes less public nuisance, and its power generating efficiency is not influenced by the scale of the equipment.
Particularly, since the solid oxide fuel cell (hereinafter referred as SOFC) operates at high temperatures of 1,000.degree. C. or more, activity of electrodes is extremely high. Thus, catalyst of a noble metal such as expensive platinum is not necessary. In addition, since the SOFC has low polarization and relatively high output voltage, its energy conversion efficiency is conspicuously higher than that in the other fuel cells Furthermore, since their constituent materials are all solid, the SOFC is stable and has a long use life.
FIG. 13 is a partial perspective view of an example of the conventional hollow-cylindrical SOFC element (an electric cell), and FIG. 14 is a sectional view taken along a line (XIV--XIV) of FIG. 13.
In FIG. 13, an air electrode 5 is formed on an outer periphery of a hollow-cylindrical porous ceramic tube 1, and a solid electrolyte 4 and a fuel electrode 3 are arranged around the outer periphery of the air electrode 5 in this order. Furthermore, an interconnector 2 is arranged on the air electrode 5 at an upper side region of the SOFC shown in FIG. 13 and a connecting terminal 6 is attached thereto. In series connection of the hollow-cylindrical SOFC elements, the air electrode 5 is connected to the fuel electrode 3 of an adjoining SOFC element through the interconnector 2 and the connecting terminal 6. In parallel connection of the hollow-cylindrical SOFC elements, the fuel electrodes 3 of the adjoining SOFC elements are connected through Ni felt or the like.
When operating the hollow-cylindrical SOFC, as shown in FIG. 14, fuel gas such as hydrogen, carbon monoxide etc., is fed around the outer surface of the fuel electrode 3 as shown by arrows D, and an oxidizing gas is fed into an oxidizing gas-flow route 8, which is an internal space 7 of the cylindrical element, as shown by arrows A and B.
However, in the internal space 7, the oxidizing gas flows in regular flow lines and in layers Therefore, in the inner peripheral portion of the porous support tube 1 facing the internal space 7, oxygen contained in the oxidizing gas is consumed successively from one end toward the other end as shown by arrows C. Consequently, as the gas-flow approaches the right end of the SOFC element shown in FIG. 14, the concentration of oxygen in the gas decreases to inactivate the ion reaction at the electrode and to lower the elevation of the temperature. Furthermore, the lowering of the temperature further inactivates the reaction. As a result, oxygen contained in the oxidizing gas flowing through the flow route 8 is not fully consumed or utilized thus lowering the power generating efficiency of the cell. Moreover, the temperature gradient between the areas of higher reactivity and lower reactivity causes large thermal strain and stress in the longitudinal direction of the SOFC element.
In addition, since the oxidizing gas flows in layers as described above, the oxidizing gas flowing in the central portion of the internal space hardly contributes to the power generation. Moreover, the oxidizing gas flows more slowly in the outer peripheral portion of the internal space 7 on the one hand and more rapidly in the central portion on the other hand, so that a larger quantity of oxygen flows through the internal space 7 before contributing to the power generation.
Similar problems occur in the case of the hollow-cylindrical SOFC in which a fuel electrode is arranged inside the solid electrolyte and the fuel gas is fed into the internal space for the power generation. In such a case, a large amount of the fuel gas flows through the internal space before contributing to the power generation.
Besides, when the hollow-cylindrical SOFC elements are electrically connected in series and in parallel, similar problems occur in the external spaces of the hollow-cylindrical elements.
FIG. 15 is a front view showing a part of a SOFC generator comprising an arrangement of such hollow-cylindrical SOFC elements, and FIG. 16 is a cross sectional view taken along a line XIV--XIV of FIG. 15.
The air electrode 5 is formed on the outer periphery of the hollow-cylindrical porous ceramic tube 1, and the solid electrolyte 4 and the fuel electrode 3 are arranged around the outer periphery of the air electrode 5 in this order. Furthermore, the interconnector 2 is arranged on the air electrode 5 at an upper side region of the SOFC element shown in FIG. 15 and the connecting terminal 6 is attached thereto. Then, the air electrodes 5 of the thus composed hollow-cylindrical SOFC elements 42 are electrically connected to the fuel electrodes 3 of the adjoining SOFC elements 42 in the upper direction shown in FIG. 15 through the interconnectors 2, the connecting terminals 6 and metal felts 50. A plurality of the hollow-cylindrical SOFC elements 42 are thus electrically connected in series in a vertical direction shown in FIG. 15. Besides, the fuel electrodes 3 of the hollow-cylindrical SOFC elements 42 adjoining each other in a horizontal direction in FIG. 15 are electrically connected to each other through metal felts 49. A plurality of the hollow-cylindrical SOFC elements 4 are electrically connected in parallel in the horizontal direction shown in FIG. 15.
When operating the cylindrical SOFC, the oxidizing gas containing oxygen is fed into the internal spaces 7 of the elements 42. Furthermore, fuel gas such as hydrogen, carbon monoxide, etc. is fed into an external space 45 formed between the outer surfaces of the arranged hollow-cylindrical SOFC elements 42 and around the outer surfaces of the fuel electrodes 3 as shown by arrows G and I in FIG. 16.
However, in the external space 45, the fuel gas flows in regular flow lines and in layers as shown by the arrows I. Therefore, carbon monoxide or hydrogen is consumed successively at the fuel electrodes 3 near the outer peripheral portion of the external space 45 from one end toward the other end. Consequently, as the gas-flow approaches the right end of the SOFC element shown in FIG. 16, the concentration of fuel ingredient in the gas decreases to inactivate the electrochemical reactions and to lower the elevation of the temperature. Furthermore, the lowering of the temperature further inactivates the reaction at the electrodes Moreover, since a large amount of CO.sub.2, steam, etc. is contained in the fuel gas having its concentration reduced, these ingredients attach to the surface of the fuel electrodes to obstruct the reaction. Thus, the reaction becomes more inactive and the temperature is lowered further. This tendency becomes more considerable as each cylindrical SOFC element is lengthened. As a result, the fuel ingredient contained in the fuel gas is not fully utilized to the reaction at the electrodes contributing to the power generation, and the power generating efficiency of each cell is lowered. Moreover, the temperature gradient between the areas of higher reactivity and lower reactivity causes large thermal strain and stress in the longitudinal direction of the SOFC element. In addition, since the fuel gas flows in layers as described above, the fuel gas flowing in the central portion of the external space 45 hardly contributes to the power generation. Moreover, the fuel gas flows more slowly in the outer peripheral portion of the external space 45 on one hand and more rapidly in the central portion on the other hand, so that a larger quantity of the fuel ingredient flows through the external space before contributing to the power generation to further lower the power generating efficiency.